bpencilgeek did a dyno, 30whp from 91 vs. ~98 octane without a tune. one of the local bay area guys art did it as well, dynoed over 400whp with N/A bolt ons whereas he got a lower dyno reading on 91.
as the posters above said, benefits start to diminish around 96ish octane
i have personally tried it with great results, several tanks of it. as stated earlier, better response in lower RPMs and smoother take off when engine is cold. higher rpm, hard to notice without a stopwatch but the difference certainly isn't as great as it is down low. most people complaining about low end torque are from california, probably because there are a lot of m3's in california but also likely due to the lower fuel quality.
this car adapts fairly quickly. e46 m3 would take almost a tank to adapt to 100 but this one seems to realize the benefit by about 1/8 to 1/4 tank.